1. Field of the Invention
The present invention relates to a method of producing p-type amorphous silicon carbide, and more particularly relates to a method for producing boron-doped p-type hydrogenated amorphous silicon carbide (hereinafter referred to as a-SiC:H), which is used as a window layer material for a p-i-n-type amorphous silicon solar cell.
2. Description of the Related Art
Use of a a-SiC:H thin film as a window layer material for a pin-type amorphous silicon solar cell has remarkably contributed to increasing the optoelectric conversion efficiency of the cell.
FIG. 2 illustrates an example of a structure of a conventional amorphous silicon solar cell. On a glass substrate 21, are laminated a tin oxide (SnO.sub.2) thin film 22, a p-type a-SiC:H layer 23, an i-type hydrogenated amorphous silicon (hereinafter referred to a a-Si:H) layer 24, an n-type a-Si:H layer 25, and an aluminum electrode 26.
The tin oxide thin film 22 of such a conventional cell is formed by vacuum evaporation or thermal CVD, while the aluminum electrode 26 is formed by vacuum evaporation or sputtering. The a-Si:H layer is formed by plasma CVD in which a mono-silane gas (SiH.sub.4) is decomposed by glow discharge at a substrate temperature of about 250.degree. C.
In the formation of the n type a Si:H layer 25, phosphine gas (PH.sub.3) is mixed with the mono-silane gas at a flow rate ratio of about 1% to the mono-silane gas. In the formation of the p-type a-SiC:H layer 23, methane gas (CH.sub.4) and diborane gas (B.sub.2 H.sub.6) are mixed with the mono-silane gas.
When light 27 is made incident onto the glass substrate 21 of such an amorphous silicon solar cell as described above, a positive electric potential is generated in the p-side tin oxide electrode 22 and a negative electric potential is generated in the n-side aluminum electrode 26, respectively, by the photovoltaic effect in the p-i-n junction. FIG. 3 shows the current-voltage characteristics of the solar cell under sun light having intensity of 100 mW/cm.sup.2. This presented by Tawada et al. in the Journal of Applied Physics, Vol. 53 (1982), pp. 5273-5281. The black dot represents the point at which a product of the output current density and the output voltage, the output electric power per square centimeter of the amorphous silicon solar cell, becomes maximum. The output power at this point is generally called the output of the amorphous silicon solar cell, and the ratio of the output electric power to the incident light power is called the conversion efficiency.
In order to increase the output power of such an amorphous silicon solar cell, it is important that the light transmissivity of the p-type layer is high so that as much light as is possible can reach the i-type layer because only the i-type layer is photovoltaically active. Therefore, it is necessary to reduce the optical absorption coefficient of the p-type layer. The optical absorption coefficient .alpha. of the p-type layer is calculated from the following expression (1) in terms of its band gap Eg (unit:electron volt): EQU .alpha.=B.sup.2 (E-Eg).sup.2 /E (E&lt;Eg) (1)
In the above expression (1), B.sup.2 represents a constant, and E represents photon energy measured in electron volts. From the expression (1), it is understood that increasing Eg reduces the absorption coefficient u. In the journal mentioned above, Tawada et al. report that the conversion efficiency of an amorphous solar cell is improved from 5-6% to about 8% by forming the p-type layer with an a-SiC:H which has a wider band gap than an a-Si:H.
As described above, the transmissivity of the p-type layer increases with an increase in band gap. However, its electric conductivity under light irradiation (photoconductivity) decreases as the band gap increases. FIG. 4 shows the dependence of the photoconductivity of an a SiC:H film on its band gap, which dependence is plotted from the results reported in the foregoing journal article by Tawada et al. Such decrease in the photoconductivity will result in increasing the series resistance of the solar cell. Generally, the thickness of the p-type layer is about 10.sup.-6 cm (10 nm) and its photoconductivity is 10.sup.-6 S/cm. This value of photoconductivity is the minimum permissible because the value corresponds to a series resistance component of 1 .OMEGA. for an amorphous silicon solar cell having an area of 1 square centimeter and the series resistance larger than 1 .OMEGA. is not desirable for the cell. FIG. 4 suggests that the band gap must be under 2.0 electron volts in order to obtain a photoconductivity larger than 10.sup.-6 S/cm. Consequently, it is difficult to provide a p-type window layer, .material for a p-i-n type amorphous silicon solar cell using an a-SiC:H film which has a band gap of 2.0 electron volts or more for improved light transmittivity as well as a photoconductivity of 10.sup.-6 S/cm or more.